In recent years, due to improvement in sound insulation inside vehicles, it is becoming more difficult to hear horns of other vehicles and sirens of emergency vehicles. Furthermore, since people more frequently wear audio equipment that has advanced sound-proof properties, such as headphones for portable music players or earphones for cellular phones, it is also becoming more difficult to hear horns of vehicles, announcements on a train, and the sound of trains passing platforms in stations. Thus, with the improvement of sound insulation inside vehicles and wearing of the audio equipment that has advanced sound-proof properties, the problem that the user has difficulties in hearing important ambient sound signal is recently increasing.
In order to solve this problem, various techniques have been suggested previously (for example, refer to Patent References 1 to 3).
FIG. 10 is a diagram showing a configuration of the first conventional technology described in the Patent Reference 1.
As shown in FIG. 10, the conventional audio identifying device is a device that transmits/reproduces, to the closed space, audio information arising in the external space, and includes: an audio collecting unit 1 which converts sound in an external space to an electric signal s; an audio reproduction unit 2 which is located in a closed space and reproduces the audio by converting the electric signal s; a signal control unit 3 which blocks/passes the electric signal s; a feature extraction unit 4 which extracts a significant feature from the electric signal s; a condition storage unit 5 which stores a feature condition of the signal; and a feature judgment unit 6 which judges whether or not the condition is satisfied by comparing the signal converted by the audio collecting unit 1 with the feature condition stored in the condition storage unit 5.
Then, when the feature judgment unit 6 judges that a predetermined condition is not satisfied, the signal control unit 3 blocks the signal, and when judging that the condition is satisfied, the signal control unit 3 passes the signal so that the user can hear only necessary audio which satisfies the condition.
Furthermore, according to the second conventional technology described in the Patent Reference 2, in an audio reproducing device installed in moving objects such as vehicles, a vehicle position detection unit detects a current position, and a volume/frequency automatic adjusting unit controls reproduced audio outputted by an output device, using the acoustic feature information stored in a volume/frequency feature storage unit, depending on the detected current position. With this, the audio reproducing device is configured such that the reproduced audio which is outputted inside a moving object may have an optimal acoustic feature according to the current position. For example, in the case where the vehicle position detection unit detects the current position as the vicinity of a railroad crossing, the audio reproducing device can lower the volume of a vehicle audio system installed in the vehicle so that the user can easily hear a railroad crossing alarm.
Furthermore, the third conventional technology described in the Patent Reference 3 provides a signal identifying circuit which detects a signal issued by emergency vehicles, alarms, or the like, and an audio quality circuit which judges a distance between a vehicle and a target audio source, and change in the distance. Judging from the distance to the target audio source and a degree of emergency, devices are suggested which automatically adjust the volume of audio equipment that has advanced sound-proof properties such as vehicle audio systems and headphones for portable music players.
Patent Reference 1: Japanese Laid-Open Patent Application No. 7-95681
Patent Reference 2: Japanese Laid-Open Patent Application No. 2002-344268
Patent Reference 3: Japanese Laid-Open Patent Application No. 7-327295